1. Field of the Invention
The present invention generally relates to implantable medical devices and more specifically relates to a system and method for monitoring master clock oscillator operation in microprocessor and state machine based implantable pulse generators and for modifying the master clock oscillator output signal in response to a low power intelligent feedback signal from the monitoring system.
2. Description of the Prior Art
Early pulse generators such as cardiac pacemakers were asynchronous in operation, providing stimulating pulses to the heart at a fixed rate independent of the physiologic demand of the patient. In recent years, pacemakers which vary the pacing rate in response to numerous patient parameters have become widely available. For example, a rate responsive pacemaker may be responsive to patient physical activity. Such a pacemaker includes a sensor which produces an output that varies between a maximum sensor output level and a minimum sensor output level and provides for a pacing rate which typically varies between a selectable lower pacing rate and an upper pacing rate. Such a pacemaker which utilizes a piezoelectric transducer, is disclosed in U.S. Pat. No. 4,485,813, issued to Anderson et al., and assigned to Medtronic, Inc.
It has become common practice in recent years to provide programmable pacemakers in order to permit the physician to select and adjust the desired parameters to match or optimize the pacing system to the heart patient's physiologic requirements in an effort to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker. Such pacemakers typically are microprocessor based systems or are based on state machines requiring accurate clocking, or timing signals. These timing signals are used to synchronize the various electronic functions within the pacemaker. Generally, a single master clock source, such as a clock oscillator, is employed to provide a periodic clocking signal at a single frequency. This clocking signal can be divided by various numerical values to result in a plurality of separate clock signals, all of which are at a fraction of the master clock signal for coordinating various handshaking operations within the programmable pacemaker.
Accurate clock signals are essential for proper functioning of the programmable pacemaker. If the frequency of the master clock signal deviates from its predetermined clock frequency, the pacemaker will not perform in the manner as intended. For example, the master clock circuit, typically including a piezo-electric crystal, may be susceptible to a noisy environment in certain rare circumstances including EMI and Cautery, and as a result not be able to guarantee the master clock signal that is delivered to the programmable pacemaker system meets the clock duty cycle requirements. In these rare cases, if the master clock signal is too narrow either in the positive or the negative half of the signal, the microprocessor, telemetry port and programmable pacemaker itself may fail to function properly. Thus, a failure at any time will result in operational failure where any programmable pacemaker block is state machine based.